Polyurethanes (PU) are well-known. They are a unique class of polymers that have a wide range of applications because their properties can be readily tailored by the variations of their components. Conventionally, PUs are extensively used as foams, coatings, adhesives, elastomers and fibers. PUs are also widely used as biomedical materials. PU hydrogels have applications as contact lenses, surgical implants, and separation membranes.
Curing by radiation, such as UV curing, saves energy and reduces or eliminates solvent emission in comparison with solvent-based systems because most formulations are 100% reactive oligomers and diluents. A UV-curable system is typically composed of reactive urethane oligomers, reactive diluents, and photoinitiators. Reactive urethane oligomer is the most important component in determining the ultimate physical properties of UV-cured coatings. Consequently, there is an increased demand for novel radiation curable PU precursors, not requiring the use of solvents.
Hydrogels have the potential to be useful in biomedical applications. However, one difficulty associated with hydrogel forming compositions is that certain compositions may worsen tissue inflammation at the site of administration. A possible explanation for this effect is that photo-initiators present in the composition may adversely affect tissues. Furthermore, inflammation can also be caused by the polymer constituting the hydrogel.
Techniques for making hydrogels are well-known. U.S. Pat. No. 1,506,471 describes a process for producing water swellable, cross-linked hydrogel polymer fibers, tapes or ribbons by extruding or spinning a solution of an isocyanate terminated prepolymer comprising the reaction product of a poly(alkyleneoxy)polyol having an average molecular weight up to 25000 g/mol and an organic diisocyanate into a cross-linking bath where said prepolymer is cross-linked by a crosslinking agent for the isocyanate terminated prepolymer which is either water or an organic polyamine. In this process the concentration of the crosslinking agent in the bath and the solvent choice are critical. Too high a concentration of cross-linker will lead to a simple end-capping reaction yielding a water soluble polymer rather than the desired hydrogel fibre. On the other hand, if the concentration of the cross-linker in the bath is too low, the product will have free isocyanate groups and the polymer will not be adequately cross-linked; this product will therefore not have adequate hydrogel properties. The bath solvents used are organic solvents including aliphatic and aromatic solvents. It would be useful if solvents could be avoided in a hydrogel fibre spinning process.
EP 0 656 077 discloses a method of producing water-absorbent fibre or film based on a water-soluble polymer starting material. The polymer is dissolved in water to form an aqueous solution which is then extruded into a heated gaseous environment. The polymer is cross-linked under the action of heat. After cross-linking the fibre is treated with moist air to raise the moisture content of the fibre. Without the crosslinking step the fibre or film will re-dissolve should it ever encounter water subsequently. The crosslinking step stabilizes the fibre or film so that on encountering water it absorbs the water without dissolving. However the more the material is heated and cross-linked, the less capable the product is of absorbing water. In addition, heating to a temperature in the range 125-250° C., is energy consuming. Furthermore, the temperature may damage the structure of the fibre or film caused by rapid vaporization of moisture at the high temperatures used for crosslinking
It would be useful to develop novel PU materials that can be cross-linked without use of solvents or heat. Advantageously the PU materials are precursors for hydrogels and can be processed by methods which were previously inaccessible. The present invention aims to resolve at least some of the problems mentioned above.